1. Field of the Invention
The present invention relates to a nickel-free white or yellowish copper alloy having excellent strength, hardness, ductility, workability, and corrosion resistance, as well as a high quality of white or yellow color tone. The copper alloy is non-allergenic and therefore is suitable for use, for example, in elements, sliders, stoppers and the like for slide fasteners, or for ornamental implements such as metallic buttons, clothing fasteners, and the like.
2. Description of the Background Art
Copper alloys that have conventionally been used, for example, in the above-mentioned fasteners include copper-nickel-zinc alloys such as nickel silver that has a white alloy hue, or copper-zinc alloys as represented by red brass, brass, and the like. However, although the nickel-containing nickel silver has excellent corrosion resistance, when it is used in a slide fastener, for example, it causes the problem of nickel allergy because such a fastener often comes in contact with the skin. On the other hand, while the nickel-free copper-zinc alloys as represented by red brass or brass do not pose the problem of nickel allergy, they tend to present a yellowish hue.
Zinc used as the alloy element for such alloys acts to increase the alloy strength, hardness, and uniform deformation through solid solution. Moreover, since zinc is cheaper than copper, it contributes to the provision of an inexpensive alloy having excellent characteristics. Meanwhile, nickel silver has excellent corrosion resistance thanks to the nickel contained therein.
However, the presence of elemental zinc in copper tends to significantly degrade corrosion resistance of the material, causing the problem of season cracking as a result of residual working strain, particularly when a copper alloy rich in zinc content is cold-formed and planted onto a base fabric to make a fastener. Meanwhile, although nickel as an alloy element contained in nickel silver acts to improve the working stress resistance over copper-zinc alloys, it is more expensive and poses the problem of allergic reactions. It is noted that there have been numerous references reporting cases where problems due to dezincing can be improved by adding small quantities of arsenic or tin. There have also been reports that annealing has resulted in significant improvement in stress corrosion cracking, or further that an extended stress corrosion cracking life has been enabled by addition of minute quantities of various elements.
However, it has generally been considered in the prior art that there are few effective means of preventing season cracking which occurs even without a stress load depending on conditions of use, when the material is cold formed with residual strain.